Hydroxyapatite (HA) is considered to be one of the best, if not the best scaffolds for bone growth. However, it is slow to resorb (and has low water solubility), and for that reason there is much interest in alternative materials that resorb faster than HA but still provide good biological properties. One approach is to use alternative calcium phosphates that dissolve more rapidly, such alternative materials including TCP, or mixtures of HA and TCP (biphasic). Another approach that can be used alone or combined with other chemistries is to alter material physical structure to increase the surface area—such as by introducing porosity, and/or by lowering the sintering temperature to reduce density.
Yet other approaches use non-calcium phosphate materials such as calcium sulfate or calcium carbonate. Carbonate-substituted HA is normally found in nature, and the carbonate can replace some of the hydroxide on the calcium hydroxide component and/or substitute some of the phosphate groups with carbonate. This material is very biologically compatible, but is available only as powder because it cannot be sintered to a high density, as sintering above about 825 degrees transforms the carbonate groups to oxide groups. Accordingly, HA and calcium carbonate have been used together as layered materials. One such commercial material has an outer, hydrothermally formed HA layer on a calcium carbonate core. An operative theory is that the HA outer layer provides initial stability and accelerated bone bonding in the graft site, while the calcium carbonate core ensures rapid remodeling once the thin outer layer of HA goes away. Alternatively, calcium sulfate can be used alone as a bone grafting material. However, it is not as osteoconductive as calcium phosphate materials, and—unlike HA—tends to resorb too quickly in many situations. Calcium carbonate can also be used alone as a bone grafting material, but it has many of the disadvantages of calcium sulfate including excessively rapid dissolution.